1. Field of the Invention
The present invention relates to an exposure apparatus including an optical system for illuminating an original to project the pattern of the original onto a substrate and a method of manufacturing a device using the exposure apparatus.
2. Description of the Related Art
An exposure apparatus is employed in a process of manufacturing a semiconductor device or a display device using photolithography. The exposure apparatus projects a pattern formed on an original (also called a reticle or a mask) onto a substrate (e.g., a wafer or a glass plate) by a projection optical system to expose the substrate. The substrate is coated with a photosensitive agent (resist). The pattern of the original is transferred onto the photosensitive agent as a latent image. A physical pattern (resist pattern) is formed on the substrate by developing the photosensitive agent.
A minimum feature size (resolution) that the exposure apparatus can form is proportional to the wavelength of the exposure light and is inversely proportional to the numerical aperture (NA) of the projection optical system. According to this principle, the shorter the wavelength of the exposure light, and the higher the NA, the better the resolution. To keep up with the recent demand for advances in micropatterning of semiconductor devices, it is demanded to further improve the resolution.
To shorten the exposure wavelength, a light source of an exposure apparatus has changed from a KrF excimer laser (wavelength: about 248 nm) to an ArF excimer laser (wavelength: about 193 nm). At present, an F2 laser (wavelength: about 157 nm) and an EUV (Extreme UltraViolet) light source are under development, aiming at their practical application as the next-generation light sources.
Under the circumstances, immersion exposure is attracting attention as a method of further improving the resolution while utilizing an ArF excimer laser or an F2 laser as a light source. The immersion exposure is a technique of further increasing the NA of a projection optical system by using a liquid as the medium of the projection optical system on the substrate side (on the image plane side). That is, the immersion exposure employs the fact that the NA of the projection optical system is given by NA=n·sin θ, where n is the refractive index of the medium. Accordingly, the NA of the projection optical system can be increased to n by filling the space between the projection optical system and the substrate with a medium (liquid) having a refractive index (n>1) higher than that of air. In other words, the immersion exposure is a technique of improving the resolution by increasing the NA of the projection optical system on the substrate side.
An exposure apparatus includes a plurality of photosensors which receive exposure light. Based on the outputs from these photosensors, the exposure apparatus performs various types of mechanical adjustment and optical adjustment, and determines various kinds of operation conditions, thereby optimizing substrate exposure. Using photosensors, an exposure apparatus can measure, for example, the σ value (effective light source distribution) of an illumination optical system and the pupil transmittance distribution of a projection optical system (Japanese Patent Laid-Open No. 2006-108689).
Precise imaging simulation can be performed by combining the measured σ value of the illumination optical system and the pupil transmittance distribution of the projection optical system with data on, e.g., a wavefront aberration measurement device for the projection optical system. This makes it possible to optimize the exposure condition of an exposure apparatus including an illumination optical system and a projection system.
By virtue of the recent technical advances of exposure apparatuses, a resolution of several tens of nanometers has become achievable. As the resolution improves, specifications that have been conventionally considered non-problematic in precision may require measurement with high accuracy. Examples of such specifications are the σ value of an illumination optical system, and the pupil transmittance distribution of a projection optical system.
An exposure apparatus has its atmosphere purged by, e.g., an inert gas to prevent impurities from adhering on its optical components. However, the long-term use of an exposure apparatus may lead to changes in σ value of an illumination optical system and in pupil transmittance distribution of a projection optical system due to contamination and deterioration of optical elements. An increase in resolution of an exposure apparatus inevitably makes changes in σ value of an illumination optical system and in pupil transmittance distribution of a projection optical system fall outside given tolerances.
Assume an exposure apparatus which mounts a measurement device for measuring the σ value of an illumination optical system and the pupil transmittance distribution of a projection optical system. In this case, the characteristics of the measurement device may change in response to changes in σ value of the illumination optical system and in pupil transmittance distribution of the projection optical system. As a consequence, changes in the σ value of the illumination optical system and the pupil transmittance distribution of the projection optical system, both of which are measured by the measurement device, may include changes in characteristics of the measurement device. To prevent this, a measurement device is calibrated as mounted in an exposure apparatus. Unfortunately, no techniques for such calibration are provided until now. Note that a measurement device for measuring the σ value of an illumination optical system and the pupil transmittance distribution of a projection optical system includes an image sensor including a two-dimensional array of a plurality of pixels. Note also that calibration of the measurement device may require calibration of the characteristics of the image sensor.